The present invention relates to an evaporated fuel gas purging system to prevent evaporated fuel gas produced in a fuel tank of a gasoline engine from purging to the atmosphere, and more particularly relates to a diagnostic method and a device for an evaporated fuel gas purging system suitable for accurately detecting leakage in the evaporated fuel gas purging system.
An evaporated fuel gas purging system is provided in a gasoline engine to prevent evaporated fuel gas produced in a fuel tank of a gasoline engine from purging to the atmosphere.
In this system, the evaporated fuel gas is temporarily absorbed to absorbent in a canister, and the absorbed evaporated fuel gas is purged into an intake pipe of the engine together with the fresh air sucked from an air port of the canister to be burned depending on the operating condition of the engine.
The evaporated fuel gas purging system sometimes becomes nonfunctional during driving of a vehicle due to various causes. When a hole or crack is formed or a pipe is uncoupled in the fuel tank or in the passage of the evaporated fuel gas between the fuel tank and the canister, it is natural that the evaporated fuel gas is not absorbed to the canister but purged to the atmosphere.
Further, since the evaporated fuel gas having been absorbed to the canister cannot be purged into the intake pipe of the engine, the evaporated fuel gas is gradually accumulated in the canister and purged to the atmosphere when the amount of the absorbed evaporated fuel gas exceeds the absorbable limit.
In order to prevent air pollution due to such a failure in the evaporated fuel gas purging system, there is proposed a device for detecting of and warning of a leakage of evaporated fuel gas to a driver during driving a vehicle.
A device for diagnosing a leakage in an evaporated fuel gas purging system is proposed, for example, in Japanese Patent Application Laid-Open No. 6-10779 (1994) where an open/close valve is provided to open or close an air port of a canister.
In this device, the inside of the evaporated fuel gas purging system including the fuel tank is brought to a negative pressure state by closing the open/close valve in the air port and opening a purge control valve and then closing the purge control valve. Leakage is detected from change of the pressure in the system under the closed state.
Another device is proposed, for example, in Japanese Patent Application Laid-Open No. 5-272417 (1993) where the inside of the evaporated fuel gas purging system is actively pressurized and a certain amount of air is injected in the system, and then the time interval in which the pressure reduces to a preset pressure is detected using a pressure switch.
However, in order to diagnose the leakage of the evaporated fuel gas purging system, pressure change due to leakage caused by the pressure difference to the atmospheric pressure is detected whichever method, depressurizing or pressurizing the closed system, is employed.
Therefore, if the pressure fluctuation occurs due to any other cause inside or outside the system, a mistake is made in the leakage diagnosis.
If evaporated fuel gas is generated inside the fuel tank, the pressure inside the evaporated fuel gas purging system increases. Since the pressure change generally occurs during the diagnosis and cannot be distinguished from the pressure change due to leakage, an error occurs in the diagnostic result.
Especially, under a circumstance in which fuel evaporation is accelerated, for example, after driving the vehicle at a high load for a long time or after leaving the vehicle in a hot place for long time when the amount of fuel remaining in the tank is small, the diagnosis becomes difficult since the pressure rise becomes extremely large by an increase in generation of evaporated fuel gas due to temperature rise of the fuel.
Further, the pressure change is different depending on the remaining amount of the fuel in the tank even if the leakage area is the same.
Furthermore, when fuel having a different volatility is supplied to the vehicle, an error occurs in the diagnosis since the generating rate of evaporated fuel gas is different even if the remaining amount of fuel is the same and consequently a difference occurs in the pressure rise.
Furthermore, the external circumstance of the evaporated fuel gas purging system, that is, a change in atmospheric pressure, also becomes a problem. A difference in the pressure change is caused between a low altitude and a high altitude exceeding 2000 m even if the leakage area is the same.
In the leakage diagnosis utilizing pressure change as described above, an error occurs in the diagnosis or a difficulty occurs in the diagnosis due to the pressure fluctuation factors inside and outside of the evaporated fuel gas purging system other than the leakage.
In the diagnosis of an evaporated fuel gas purging system, an object of the present invention is to achieve an accurate diagnosis by providing a reference leakage apparatus (a gauge) having a preset amount of leakage, by intentionally causing a known leakage using the gauge under the same condition of diagnosing a leakage, and by comparing the pressure change with the pressure change in using the reference leakage apparatus.
In the process of diagnosing leakage by depressurizing or pressurizing and closing the inside of the evaporated fuel gas purging system and leaving it under the presence of pressure differences to the atmospheric pressure and then by detecting the pressure change, a difference occurs in the pressure changes inside the evaporated fuel gas purging system between opening and closing of the gauge.
It is possible with the present invention to obtain improved sensitivity of the difference in the pressure changes to the leakage under the condition during diagnosis such as fuel temperature, atmospheric pressure, amount of remaining fuel, fuel properties and so on.
An object of the present invention is to improve the accuracy of leakage diagnosis of the evaporated fuel gas purging system by removing the effect due to evaporation of the fuel inside the tank.
Another object of the present invention is to provide a gauge so that in the case of the intentional leaking of evaporated fuel gas, the leaked evaporated fuel gas is not purged to the atmosphere, but leaked from the closed evaporated fuel gas purging system to an intake portion of an engine (for example, the portion between an air cleaner and a throttle valve) having a pressure near the atmospheric pressure.
In the case of performing the diagnosis by pressurizing, the evaporated fuel gas leaking through the gauge is sucked to the cylinders from the upstream of the throttle valve together with intake air. Therefore, the evaporated fuel gas is burned and consequently is not purged to the atmosphere.